1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns a method and apparatus for providing liquid reflux to a refluxed heavies removal column of a liquefied natural gas (LNG) facility.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source of supply to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
In most LNG facilities it is necessary to remove heavy components (e.g., benzene, toluene, xylene, and/or cyclohexane) from the processed natural gas stream in order to prevent freezing of the heavy components in downstream heat exchangers. It is known that refluxed heavies columns can provide significantly more effective and efficient heavies removal than non-refluxed columns. However, many existing LNG facilities were originally constructed with non-refluxed heavies removal columns. Thus, it would be desirable to retrofit existing LNG facilities employing non-refluxed heavies removal columns with refluxed heavies removal columns.
One problem with retrofitting an existing LNG facility with a refluxed heavies removal column is the lack of availability of a suitable reflux stream. The reflux stream to a heavies removal column must be a low-temperature, liquid, methane-rich stream. It is not economically feasible to use existing liquefied methane-rich steams of conventional LNG facilities as reflux to the heavies removal column because such liquid streams are typically at low pressures. A cryogenic pump would be required to transport these existing low-pressure, methane-rich streams to the heavies removal column. It is well know that cryogenic pumps are very expensive, and the cost of employing an additional cryogenic pump in an LNG facility would likely outweigh the benefits of switching from a non-refluxed to a refluxed heavies removal column.
If an existing high-pressure, methane-rich stream could be employed as the reflux stream to the heavies removal column, the need for a cryogenic pump could be obviated because the elevated pressure of the steam could be used to transport it to the heavies removal column. In existing LNG facilities, however, such high-pressure, methane-rich streams are not liquid streams, and current LNG facilities do not have the excess cooling capacity to liquefy such high-pressure, methane-rich streams.